1. Field of the Invention
The present invention relates to a thermal-transfer-type color printer which transfers colored images onto a paper by using an ink ribbon.
2. Prior Art
FIG. 1 is a drawing simply illustrating an example of a mechanical structure of a thermal-transfer-type color printer conventionally known. In FIG. 1, an ink ribbon 2, which has not been used yet, is wound around a ribbon-supply roller 1. The overall area of the ink ribbon 2 is divided into a plenty of areas in a longitudinal direction, wherein each of those areas is painted with ink having a specific color. For example, three colors (i.e., yellow color, magenta color and cyan color) of ink are sequentially and repeatedly painted on the ink ribbon 2; or four colors (i.e., yellow color, magenta color, cyan color and black color) of ink are sequentially and repeatedly painted on the ink ribbon 2.
The ink ribbon 2, supplied from the ribbon-supply roller 1, is led to a place between a thermal head 4 and a platen roller 5 by means of a guide roller 3B. A paper 6 is wound around the platen roller 5, so that the ink ribbon 2 comes in contact with the paper 6. The ink, painted on the ink ribbon 2, is melted by heat, applied by the thermal head 4, so that a specific image is transferred onto the paper 6 with a specific color. After a printing is performed, the ink ribbon 2 turns to a used ink ribbon 7, which is then led to a ribbon-winding roller 9 by means of a guide roller 8. Thus, the used ink ribbon 7 is wound up around the ribbon-winding roller 9.
The platen roller 5 is made by rubber materials and the like. This platen roller 5 is sandwiched between pinch rollers 10 and 11.
When transferring one-color ink (e.g., yellow-color ink) onto the paper 6, the platen roller 5 is driven to rotate in a forward direction. In this case, the paper 6 is held between the platen roller 5 and the pinch rollers 10, 11 which are pressed with each other; and the paper 6 is transported in a direction "m". At the same time, the ink ribbon 2, supplied from the ribbon-supply roller 1, is led to the place between the thermal head 4 and the platen roller 5 by means of the guide roller 3; and then, the used ink ribbon 7 is led to the ribbon-winding roller 9 by means of the guide roller 8 and is wound up.
In addition, the thermal head 4 is driven to move in a direction "p" by a spring (not shown), so that the thermal head 4 is pressed against the ink ribbon 2. In synchronism with a speed of carrying the ink ribbon 2 and the paper 6, heating elements of the thermal head 4 are heated in response to print data such as character data and image data. Under the effects of the heat and pressure, a specific image is transferred onto the paper 6 by one-color ink (e.g., yellow-color ink) while the paper 6 is held between the thermal head 4 and the platen roller 5. Next, in order to transfer the another-color image by using another-color ink (e.g., magenta-color ink) such that the magenta-color image overlaps with the yellow-color image, a preparation stage is made. In the preparation stage, the platen roller 5 is driven to rotate backward, so that the paper 6 is carried in a direction "n". At this stage, the thermal head 4 is rotably moved in a direction "q", while the ink ribbon 2 is wound up by the ribbon-winding roller 9 so that the ink ribbon 2 does not come in contact with the paper 6.
The above-mentioned operations are performed with respect to each of three colors of ink or each of four colors of ink. As a result, characters, images and the like are printed in a multi-colored manner on a single paper 6.
As described above, the thermal-transfer-type color printer is designed such that three colors of ink or four colors of ink are sequentially melt and transferred on a single paper 6. Hence, if a printing position of one-color image does not accurately match with a printing position of another-color image, there occurs a drawback that a desired color printing, corresponding to the print data inputted, cannot be obtained. In the worst case, a precision of the color printing is so poor that the printed characters cannot be clearly read by a person.
In order to avoid the above drawback, it is demanded that a precision of overlapping the print dots among the plural colors of ink should be sufficiently high. In the thermal-transfer-type color printer, the thermal head 4 is normally set in a certain positional state, while the ink ribbon 2 and the paper 6 are carried in a desired direction. Hence, it is demanded to improve the precision of carrying the ink ribbon 2 and the paper 6, particularly, the precision of carrying the paper 6.
In order to achieve the above-mentioned demands, it is necessary that the mutual pressure should be uniformly applied between the platen roller 5 and the pinch rollers 10, 11. In that sense, the rubber, which is used for the platen roller 5, should have a certain degree of hardness or more.
The thermal head 4 has a length which corresponds to the width of the paper (e.g., A3-size paper or A4-size paper). In other words, the thermal head 4 has a line of heating elements; thus, it is possible to simultaneously print one line of print data on the paper. Therefore, it is an essential condition that the uniform pressure should be imparted between the thermal head 4 and the platen roller 5 in the overall length. If such condition is not satisfied, the plural colors of ink cannot be certainly transferred onto the paper 6 without causing the color shade or drop-out errors in the printing.
Meanwhile, there is a limit in the degree of flatness of the heating elements of the thermal head 4 because of the manufacturing process. Further, in order to reduce the size of the thermal-transfer-type color printer, the diameter of the platen roller 5 cannot be increased so much. Therefore, when the thermal head 4 is pressed against the platen roller 5, a small deflection may be occurred on the platen roller 5.
The above-mentioned facts indicate that the rubber, used for the platen roller 5, should have a certain degree of flexibility.
As described above, the contradictory characteristics should be required for the rubber material, used for the platen roller 5. Conventionally, the degree of hardness of the rubber is set at 50.degree.. However, such degree of hardness cannot offer a good precision of overlapping the print dots among the plural colors of ink and a satisfactory thermal-transfer performance in the color printing. This disadvantage depends upon the complex functions of the platen roller 5. In the conventional printer, the platen roller 5 has a function to carry the paper 6 as well as a function to act as a support body to which the thermal head 4 is pressed in the thermal-transfer mode.
Another thermal-transfer-type color printer is provided to eliminate the above-mentioned disadvantage. This printer provides two rollers, wherein one roller is provided to perform the function to carry the paper and another roller is provided to perform the function to support the thermal head. FIG. 2 is a drawing simply illustrating a mechanical structure of this printer. In FIG. 2, the parts identical to those shown in FIG. 1 are designated by the same numerals; hence, the description thereof will be omitted. In the thermal-transfer-type color printer shown in FIG. 2, a set of the platen roller 5 and the pinch rollers 10, 11 shown in FIG. 1 is replaced by a set of a platen roller 12, paper-feed rollers 13, 14 and pinch rollers 15, 16.
The platen roller 12 is made by the rubber material which is optimum to the thermal-transfer-type printing. The degree of hardness is set around 40.degree., for example, which is the optimum degree of hardness for the thermal-transfer-type printing. This platen roller 12 is not driven in the thermal-transfer mode using the color ink. The paper-feed roller 13 is made by the rubber and the like. Another paper-feed roller 13 is called an elusion roller. A base portion of the paper-feed roller 13 is made by the stainless steel. Particles, which are made by a certain material like Ni-Cr material and each of which has a diameter of 20-60 .mu.m, are sprayed on an outer peripheral face of the paper-feed roller 14; thus, micro projections (not shown) are formed on the surface of the paper-feed roller 14.
In the thermal-transfer mode, using one color ink (e.g., yellow-color ink), of the printer shown in FIG. 2, the paper-feed roller 14 is driven to rotate forward, whereas both of the platen roller 12 and the paper-feed roller 13 are not driven. Hence, these rollers 12 and 13 act like a load to the paper-feed roller 14. Therefore, the paper 6 is carried in a direction "x" while being held between the paper-feed roller 14 and the pinch roller 16 which are pressed with each other. The ink ribbon 2, supplied from the ribbon-supply roller 1, is led to a place between the thermal head 4 and the platen roller 12 by means of the guide roller 3; and then, the used ink ribbon 7 is led to the ribbon-winding roller 9 by means of the guide roller 8 and is wound up.
The thermal head 4 is moved in the direction "p" under the effect of the spring (not shown) and is pressed against the platen roller 12. In synchronism with the speed of carrying the ink ribbon 2 and the paper 6, the heating elements of the thermal head 4 are heated in accordance with the print data such as the character data and image data. Under the effects of the heat and pressure, one-color ink (e.g., yellow-color ink) is melt and transferred onto the paper 6, which is held between the thermal head 4 and the platen roller 12.
In the printer shown in FIG. 2, the degree of hardness for the platen roller 12 is set at 40.degree., which is smaller (i.e., softer) than the degree of hardness employed by the printer shown in FIG. 1. Thus, it is possible to impart the uniform pressure between the thermal head 4 and the platen roller 12 along its overall length in the thermal-transfer mode. Therefore, the thermal-transfer performance can be improved.
In addition, the micro projections are formed on the peripheral surface of the paper-feed roller 14. Therefore, as compared to the platen roller 5 shown in FIG. 1, the paper-feed roller 14 has a larger coefficient of friction. In other words, the performance of feeding the paper can be improved.
By the way, the micro projections formed on the peripheral surface of the paper-feed roller 14 are not formed uniformly because of the manufacturing process. In other words, those micro projections are not uniform in size and arrangement. Due to the ununiformity, the pressure mutually applied between the paper-feed roller 14 and the pinch roller 16 cannot be made uniform along an overall length of the roller 14. Further, the platen roller 12 and the paper-feed roller 13 are not driven during the printing; in other words, those rollers act like the load to the paper-feed roller 14. This load may cancel an improvement in the performance of feeding the paper which is once improved by increasing the coefficient of friction.
Thus, as the plural colors of ink are transferred onto the paper, the printing positions are shifted such that the printing position of one-color ink may not accurately match with the printing position of another-color ink. Therefore, even in the printer shown in FIG. 2, the precision of overlapping the print dots among the plural colors of ink cannot be improved so much as compared to the foregoing printer shown in FIG. 1.
In the printing mode of the thermal-transfer-type color printer described above, the paper-feed roller 14 is driven to rotate forward, while the platen roller 12 and the paper-feed roller 13 are not driven so that those rollers may freely rotate in accordance with the feeding operation of the paper 6. Thus, in the printing mode, one edge of the paper 6 is drawn in the direction "x" by the roller 14, whereas another edge of the paper 6 is not drawn back in a direction opposite to the direction "x". In other words, a tension is not applied to the paper 6. For this reason, the paper 6 may be easily deflected between the roller 14 and the platen roller 12.
In general, the operations of the thermal-transfer-type color printer may be affected by the following fluctuating factors 1 to 3.
1 The roller 13 is not balanced with the platen roller 12 in terms of a degree of cylindrical shape.
2 The pinch roller 15 is not balanced with the roller 13 in terms of a pressing load.
3 The thermal head 4 is not balanced with the platen roller 12 in terms of a pressing load.
Due to the above-mentioned fluctuating factors, the deflection of the paper 6 may be occurred or an attitude of the paper 6 is inclined so that the paper-feeding operation cannot be made in a desired direction. Thus, the carried position of the paper 6 does not accurately match with a desired position, which raises a problem that the precision of overlapping the print dots among the plural colors of ink becomes lower.